Method and device for anaerobically fermenting organic material

ABSTRACT

This invention relates to a method for anaerobically fermenting biodegradable organic material, whereby this material is mixed with a quantity of already fermented material as an inoculum for the active anaerobic fermentation and this mixture is introduced at the top into a fermentation chamber ( 1 A) in which a fermenting mass is situated, which, while being fermented, moves from an inlet ( 6 A) situated at the top towards an outlet ( 8 ) situated at the bottom. Before the mixture is added to the fermenting mass, it is expanded beforehand until its density approximately coincides with the density of the already present fermenting mass at the beginning of this downward movement.

[0001] This invention relates to a method for anaerobically fermentingbiologically degradable organic material, whereby this material is mixedwith a quantity of already fermented material as an inoculum for theactive anaerobic fermentation, and whereby this mixture is introduced atthe top into a fermentation chamber in which a fermenting mass issituated, which moves from an inlet situated at the top towards anoutlet situated at the bottom.

[0002] By organic material, here in particular the organic fraction ofdomestic waste is intended, and of similar industrial waste and otherorganic fractions, such as, for example, slurry from water purificationinstallations, slurry from the paper industry, or other kinds of organicslurry.

[0003] Methods for anaerobically fermenting organic waste can be dividedinto wet and dry manners of fermentation.

[0004] With wet fermentation methods, fresh or recycled water is addedto the solid organic waste or slurry in order to form a highly liquidmash or slurry which can be pumped easily and which can easily be mixedin a fermentation tank. Such mash with 6 to 10% of dry matter, dependingon the viscosity of the starting material to be treated, is pumped intoa fermentation tank and is transformed into biogas, in mixedfermentation systems, in one or two phases and with mesophilic orthermophilic temperatures.

[0005] Due to the high water content of the material in the fermentationtank, the density in this tank is relatively uniform, and the producedbiogas can easily escape from the mixed mash.

[0006] With a dry fermentation, on the contrary, the quantity of waterwhich is added is limited, such that a relatively solid biologicallydegradable waste is pumped or pushed into a “dry” fermentation tank bymeans of special pumps. For organic fractions originating from domesticwaste, the dry matter content of the material fed into the tank isbetween 15 and 45%. For more viscous materials, this may be up to 10 to15%.

[0007] In most cases, with dry fermentation the material is pumped atthe bottom into the standing tank, whereby gas is injected in order toobtain a mixing and a horizontal movement of the material. The tank mayalso be installed horizontally and may be provided with a mixing devicewhich realizes the horizontal movement.

[0008] Such method of introducing the material at the bottom of astanding tank is described in WO 86/05200. Biogas is brought into thefermentation tank at different locations, through the underside, suchthat this underside is divided into sectors. The biogas provides for amixing of the contents of the sectors and a movement of the fermentingmass from one sector to the other and finally towards the outlet, alsosituated at the bottom.

[0009] EP-A-0.476.217 describes a method, whereby a horizontalfermentation tank with therein a mixing device is used. After havingbeen mixed with a portion of fermented material, the biodegradablematerial is pushed, through a feeding tube realized as a heat exchanger,into the tank and in this tank is mixed by a blender, as a result ofwhich the fermenting mass flows horizontally through the tank, from oneextremity to the other, where it is discharged at the bottom.

[0010] According to EP-A-0.205.721 and EP-A-0.577.209, the fermentationis performed in a vertical fermentation tank, without a mixing devicetherein. The biodegradable material is mixed with fermented material asan inoculum and is pumped into the tank at the top.

[0011] In the tank, the fermenting mass sinks, as fermenting material isdischarged at the bottom. This latter is performed by means of a slidinggrid moving to and fro above the flat bottom of the tank, which gridpushes this fermented material into a screw conveyor device situatedtherebelow. This device pushes the fermented material into another screwfunctioning as a lock by forming a stopper of fermented material at itsconical outlet.

[0012] In the method according to EP-A-0.577.209, moreover, due to thedesign of the device, a fraction division into a liquid and a solidfraction is obtained, such that the content of solid matter in thefermentation tank can be maintained and that the dry matter content ofthe fed material, which consists of a mixture of inoculum and freshmaterial, is situated between 15 and 40%.

[0013] In these fermentation tanks without mixer, it is typical that thecontent thereof rises and expands as a result of the produced biogas.This biogas can not bubble immediately upward as in a wet tank and cannot easily escape, due to the high viscosity of the material in which itis created and through which it must weave its way upward.

[0014] In practice, the material will expand as a consequence thereof,and its density will be reduced by about 10 to 40%. The average densitydepends on various parameters, such as the degree of biogas production,the feeding frequency, the kind of structure of the added material, andthe height of the feeding tank. Depending on the feeding, the densitymay vary from 0.7 to 1.2 kg/l.

[0015] It was noted that this feeding matter, being a mixture of freshbiodegradable waste and fermented material or residue, has a densitywhich mostly is higher than 1.0 kg/l, on account of the fact that theprevailing biogas during mixing of the fermented material and the freshmaterial can escape from the fermented material.

[0016] When this feeding matter with fresh material and inoculum, which,due to the degassification, has obtained a higher density, is brought atthe top into the fermentation tank in which the material has a lowerdensity, then, depending on the difference in density, the dimensionsand the diameter of the fermentation tank and the extraction systemthereof, too fast a sinking of this feeding matter through the mass inthe tank can be noted, such that this feeding matter arrives faster atthe bottom than the remainder of the fermenting mass and, thus, isdischarged from the tank without having undergone an optimumfermentation.

[0017] The invention aims at a method which does not have thesedisadvantages and whereby the mixture of fresh material and fermentedmaterial is brought at the top into a vertical fermentation chamber inwhich material moves downward from the top, however, whereby it isavoided that said added mixture sinks faster than the remainder of thefermenting mass.

[0018] According to the invention, the mixture, before being added tothe fermenting mass and further moving downward during fermentation, isexpanded beforehand until its density approximately coincides with thedensity of the already fermenting mass at the beginning of this downwardmovement.

[0019] Preferably, the mixture is beforehand expanded or reduced indensity, while being moved upward.

[0020] Preferably, a mixture is made of one quantity of fresh material,together with one to ten quantities of fermented material.

[0021] Preferably, a mixture is treated, having a dry matter content of10 to 45%.

[0022] The downward movement of the fermenting mass in the fermentationchamber can take place in a fermentation tank which is free of anymixer, whereby the mixture is supplied at the upper side of thefermentation tank, after it has been reduced in density in an expansionvessel.

[0023] The mixture can be expanded beforehand by means of a precedingbiological pre-fermentation, by injecting a gas into the mixture, bybeating this mixture or also by adding chemicals which lead to therelease of gasses in the mixture, or by a combination of two or more ofthese procedures.

[0024] If the expansion or density reduction is performed by means ofbiological pre-fermentation, this can be performed in an expansionvessel in which the mixture remains at least 5 minutes, and preferablybetween 15 minutes and 3 hours.

[0025] The sojourn time in the expansion vessel also may be more thanthree hours, for example, 3 to 72 hours.

[0026] The invention also relates to a device which is particularlysuitable for applying the method according to any of the preceding formsof embodiment.

[0027] Consequently, the invention relates to a device for anaerobicallyfermenting organic waste, which device comprises a fermentation tankwith a fermentation chamber, an outlet for fermented material at thebottom of this tank, and a biogas outlet at the top and a feeding devicecomprising a mixer and which, at the top, gives out into thefermentation tank, and which is characterized in that the feeding devicecomprises at least an expansion vessel situated between the mixer andthe fermentation tank.

[0028] The feeding device also may comprise a pump to which the mixergives out, whereby the expansion vessel is situated between the pump andthe fermentation tank.

[0029] This expansion vessel may be situated at least partially in thefermentation tank as well as at the exterior side thereof.

[0030] The expansion vessel may be reduced to a supply conduit.

[0031] The device may comprise a device for gas injection or an inletfor chemicals which lead to the release of gasses, which latter connectsto the expansion vessel, the mixer or a pump connected thereto, or abeating device can be provided in the feeding device.

[0032] With the intention of better showing the characteristics of theinvention, hereafter, as an example without any limitative character,several preferred forms of embodiment of a method and device foranaerobically fermenting organic material according to the invention aredescribed, with reference to the accompanying drawings, wherein:

[0033]FIG. 1 schematically represents a vertical cross-section of adevice for anaerobic fermentation according to the invention;

[0034]FIG. 2 represents a cross-section according to line II-II in FIG.1;

[0035]FIG. 3 schematically represents a cross-section analogous to thatof FIG. 1, however, relating to another form of embodiment;

[0036]FIG. 4 represents a cross-section according to line IV-IV in FIG.3;

[0037] FIGS. 5 to 9 represent schematic cross-sections analogous tothose of FIGS. 1 and 3, however, relating to still other forms ofembodiment of the invention.

[0038] The device for anaerobically fermenting organic material,represented in FIGS. 1 and 2, substantially consists of a closedfermentation tank 1, which comprises a fermentation chamber 1A, and afeeding device 2 comprising a transport device 3, a mixer 4, a pump 5and an expansion vessel 6, which, at the top, gives out into thefermentation tank 1. The outlet of the expansion vessel 6 at the topforms the inlet 6A of the fermentation tank 1.

[0039] The inlet for material in the fermentation tank 1, thus, issituated at the top, and the outlet 8, which can be closed off by avalve 7, is situated below the funnel-shaped bottom of this fermentationtank 1. This fermentation tank 1 also may have a flat bottom with anoutlet 8 in the bottom, possibly by means of screws of other extractionsystems, or an outlet 8 at the underside of its wall.

[0040] In the outlet 8, as represented, transport screws 9 can beinstalled. The mixer 4 is connected to this outlet 8 and is situatedbelow an extremity of the transport device 3, which, in the representedexample, is formed by a transport conveyor. By means of this transportconveyor or another transport device, such as a screw conveyor, freshorganic material can be added to the mixer 4.

[0041] Downstream of the valve 7, a branch 10, which can be closed offby a valve 11, connects to the outlet 8, between two screws 9. In thebranch 10, downstream of the valve 11, a transport screw 12 can also bearranged. The branch 10 connects to a dewatering device 13.

[0042] To the mixer 4, a water conduit 15, which can be closed of f bymeans of a valve 14, is connected for possibly supplying water to themixer 4, as well as a steam conduit 17, which can be closed off by avalve 16, for injecting steam in order to heat the material.

[0043] The outlet of the mixer 4 connects to the inlet of the pump 5,whereas the outlet of the pump 5, on one hand, connects to a conduit 19,which can be closed off by a valve 18, which conduit 19 serves fortransporting off material, and, on the other hand, connects to theunderside of the expansion vessel 6, by means of a conduit 20 whichpossibly can be closed off by a valve 20A.

[0044] The expansion vessel 6 penetrates into the fermentation tank 1through the funnel-shaped bottom thereof and, with its opening, givesout into the fermentation tank 1 above the upper side of the inlet 6A,below or above the level 21 of the fermenting mass in this tank 1.

[0045] The expansion vessel 6, for example, is round, as well as thefermentation tank 1, but has a much smaller diameter.

[0046] At the top, an outlet 22 for biogas connects to the fermentationtank 1.

[0047] Fresh material to be treated is introduced into the mixer 4 bymeans of the transport device 3, together with a part of the fermentedmaterial which is coming through the opened valve 7 out of thefermentation tank 1 and further is transported by the screws 9 oranother transport device to the mixer 4.

[0048] In the mixer 4, the fresh material and the fermented material asan inoculum are mixed in a ratio of one quantity of fresh material forone to ten quantities of fermented material.

[0049] The dry matter content of the mixture is maintained between 10and 45% and preferably between 15 and 45% and, if necessary or desired,a quantity of water, for example, waste water, is brought into the mixer4 through water conduit 15. This water can be supplied hot in order tobring the final mixture to a temperature of 30 to 42° C. (mesophilic) or45 to 60° C. (thermophilic). If no water must be added, steam can beinjected, by means of the steam conduit 17, into the mixer 4 in order tobring the temperature onto the desired level.

[0050] Subsequently, the mixture produced in the mixer 4 is pumped tothe expansion vessel 6 through conduit 20. Valve 18 is closed and valve20A is open. In this expansion vessel 6, the mixture moves upward, asindicated by arrow P1, whereas it undergoes a biologicalpre-fermentation with the production of biogas.

[0051] As a consequence thereof, the mixture will rise in the expansionvessel 6 and decrease in density. The sojourn time of the mixture inthis expansion vessel 6 is at least 10 minutes and preferably issituated between 30 minutes and 3 hours.

[0052] This sojourn time is chosen such that, when, due to the pumpingin of new mixture, the mixture is pushed out of the expansion vessel 6and arrives at the top of the fermentation tank 1, its density isapproximately equal to the density of the fermenting mass which alreadyhas been present at the top of this fermentation tank 1.

[0053] As a result of the formation of biogas, the density of thispre-fermented supplied mixture, due to rising, is lower than the densityof the fresh supplied mixture which is pumped by the pump 5 from themixer 4 into the expansion vessel 6.

[0054] In the fermentation tank 1, the further fermentation takes place,and the fermenting mass sinks towards the outlet 8, where it leaves thefermentation tank 1 as fermented material. The formed biogas isdischarged through outlet 22.

[0055] By opening the valve 11, fermented material from outlet 8 getsinto branch 10. This material is transported by the screw 12 to thedewatering device 13, where it is separated into press water 13A and apress cake 13B, which are transported off.

[0056] It is clear that the device may comprise more than one expansionvessel 6, whereby the expansion vessels 6, for example, three expansionvessels 6, are connected to the pump 5, and whereby preferably, they areequal to each other and installed in the same manner. In FIGS. 1 and 2,two additional expansion vessels 6 are represented in dashed line.

[0057] The form of embodiment represented in FIGS. 3 and 4 differs fromthat according to FIGS. 1 and 2 in that the expansion vessel 6,expansions vessels 6, respectively, if there are several, is or areprovided at the exterior side on the fermentation tank 1.

[0058] In FIG. 3, moreover in dashed line a conduit 23 is represented,which forms a bypass of the expansion vessel 6, which bypass, on onehand, connects to the conduit 19 and, on the other hand, branches off inbranches 23A which give out onto the upper side of the fermentation tank1 and can be closed off by a valve 24. The outlets of the branches inthe tank 1 form the inlet 6A of this tank 1.

[0059] The functioning is as described above, however, if desired,through bypass 23, the mixture of inoculum and fresh material can bebrought without expansion into the fermentation tank 1.

[0060] The expansion of the mixture of inoculum and fresh material doesnot necessarily have to take place by means of a pre-fermentation in anexpansion vessel 6. This expansion may also be obtained by injectinggas, for example, part of the collected biogas, into the mixture,whereby, when being introduced into the fermentation chamber 1A, themixture immediately expands, and the density thereof becomes equal tothe density of the fermenting mass at the top of the fermentationchamber 1A.

[0061] This gas can be injected into the expansion vessel 6, however,this expansion vessel 6 may also be reduced to a normal conduit, forexample, only the bypass 23 with the valves 24 of the form of embodimentaccording to FIGS. 3 and 4, whereby the gas is brought under pressureinto the mixture.

[0062] In FIG. 5, such device is represented, whereby thus to conduit23, an injection conduit 25 gives out which can be closed off by a valve26.

[0063] The same effect can be obtained by, instead of injecting gas,injecting a chemical agent which causes gas production in the mixture.In FIG. 5, in dashed line an inlet 27 of such chemical agent isrepresented, which connects to conduit 23 and can be closed off by avalve 28. This chemical agent which causes gas production, possibly canbe added in the mixer 4 or in the pump 5.

[0064] Still another possibility for reducing the density of the mixtureof fresh material and inoculum supplied to the fermentation chamber 1Aand for expanding this mixture, consists in beating this mixture, forexample, in the expansion vessel 6.

[0065] In this case, too, the expansion vessel 6 can be reduced to aconduit 23, whereby the beating can take place by beating devices 29 inchambers 30 in the branches 23A of this conduit 23, as represented inFIG. 6.

[0066] The expansion vessel 6 does not necessarily have to extend overthe entire height of the fermentation tank 1. It may, for example, be avessel which connects to the fermentation tank 1 at the exterior, athalf the height.

[0067] The device does not necessarily have to comprise a pump 5.Instead of a pump, it may comprise another transport mechanism, such asa screw or the like.

[0068] The expansion vessel 6 may also be formed by one or morecompartments 32, bordered by partitions 31, of a vertical tank 33,whereby then the space outside of the compartment 32 or the compartments32 forms, form, respectively, the fermentation tank 1 with thefermentation chamber 1A.

[0069] It is essential that the compartment 32 or the compartments 32gives out, give out, respectively, at the top of the fermentationchamber 1A, and allow for a certain upward flow of the mixture to befermented.

[0070] In FIG. 7, a device with such tank 33 is represented. A partition31, situated above the conical bottom of the tank 33, together with apart of the wall of the tank 33 forms a compartment 32 which is closedat the bottom and open at the top. The mixture of fresh material andinoculum is introduced into this compartment at the bottom, where it isexpanded by means of biological pre- fermentation.

[0071] In FIG. 8, a similar device is represented, however, in this casethe partition 31 is directed upwardly inclined, starting from the insideof the tank. As represented, this ring can be provided over the entireinner circumference of the tank 33, such that the compartment thus formsa circle-shaped gutter, or can be provided locally, such that severaltrough-shaped compartments 32 are formed at the interior side of thetank 33, which latter compartments form an expansion vessel 6.

[0072] The expansion vessel 6 may not only be situated at an interiorside of the fermentation tank 1, as represented in FIGS. 1 and 2,however, it may also be situated centrally, around the fermentation tank1, as represented in FIG. 9.

[0073] This fermentation tank 1 then is formed by a tubular elementwhich is open at the top and which protrudes with one outlet 8 throughthe funnel-shaped bottom of expansion vessel 6.

[0074] The invention is in no way limited to the forms of embodimentdescribed in the aforegoing and represented in the figures; on thecontrary, such method and device for anaerobically fermentingbiodegradable material may be realized in different variants, withoutleaving the scope of the invention.

1. Method for anaerobically fermenting biodegradable organic material,whereby this material is mixed with a quantity of already fermentedmaterial as an inoculum for the active anaerobic fermentation and thismixture is introduced at the top into a fermentation chamber (1A) inwhich a fermenting mass is situated, which, while being fermented, movesfrom an inlet (6A) situated at the top towards an outlet (8) situated atthe bottom, characterized in that the mixture, before being added to thefermenting mass and further moving downward during fermentation, isexpanded beforehand until its density approximately coincides with thedensity of the already present fermenting mass at the beginning of thisdownward movement.
 2. Method according to claim 1, characterized in thatthe mixture beforehand is expanded or is reduced in density, while beingmoved upward.
 3. Method according to claim 1 or 2, characterized in thata mixture is made of one quantity of fresh material, together with oneto ten quantities of fermented material.
 4. Method according to any ofthe preceding claims, characterized in that a mixture of biologicallyfresh material and fermented material is fermented, having a dry mattercontent of 10 to 45%.
 5. Method according to any of the precedingclaims, characterized in that the downward movement of the fermentingmass in the fermentation chamber (1A) can take place in a fermentationtank (1) which is free of any mixer, whereby the mixture is introducedat the upper side of the fermentation tank (1) after it has been reducedin density in an expansion vessel (6).
 6. Method according to any of thepreceding claims, characterized in that the mixture is expandedbeforehand by means of one or more of the following techniques: apreceding biological pre-fermentation, the injection of a gas into themixture, the beating of- the mixture, the addition of chemicals whichcause the formation of gas in the mixture.
 7. Method according to claim6, characterized in that the expansion or density reduction is performedby a biological pre-fermentation, and that they are performed in anexpansion vessel (6) in which the mixture remains at least 5 minutes andpreferably between 15 minutes and 3 hours.
 8. Method according to claim6, characterized in that the mixture's sojourn time in the expansionvessel (6) during expansion is 3 to 72 hours.
 9. Device foranaerobically fermenting organic waste, which comprises a verticalfermentation tank (1), an outlet (8) for fermented material at thebottom of this tank (1), and an outlet (22) for biogas at the top, and asupply device (2) which comprises a mixer (4) and which gives out intothe fermentation tank (1) at the top thereof, characterized in that thesupply device (2) comprises at least one vertical expansion vessel (6)which is situated between the mixer (4) and the fermentation tank (1).10. Device according to claim 9, characterized in that the supply device(2) comprises a pump (5) to which the mixer (4) gives out, whereby theexpansion vessel (6) is situated between the pump (5) and thefermentation tank (1).
 11. Device according to claim 9 or 10,characterized in that the expansion vessel (6) is situated at leastpartially in the fermentation tank (1).
 12. Device according to claim 9or 10, characterized in that the expansion vessel (6) is installed atthe exterior side of the fermentation tank (1).
 13. Device according toclaim 9 or 10, characterized in that the expansion vessel (6) is acompartment (32) formed by at least one partition (31) and the interiorwall of a tank (33), whereby the chamber of the tank (33) next to thecompartment forms the fermentation tank (1).
 14. Device according toclaim 9 or 10, characterized in that the expansion vessel (6) completelysurrounds the fermentation tank (1).
 15. Device according to claim 9 or10, characterized in that the expansion vessel (6) is reduced to aconduit (23).
 16. Device according to any of the claims 9 to 15,characterized in that it comprises an injection device (25-26) for gas,or an inlet (27) for at least a chemical agent which causes gasproduction, which connects to the expansion vessel (6), to the mixer (4)or to a pump (5) connected to the mixer (4).
 17. Device according to anyof the claims 9 to 16, characterized in that a beating device (29) isprovided in the expansion vessel (6).
 18. Device according to claims 15and 17, characterized in that the conduit (23), by means of branches(23A), gives out onto the upper side of the fermentation tank (1) andthat in the branches (23A), chambers (30) are formed in which a beatingdevice (29) is provided.
 19. Device according to any of the claims 9 to19, characterized in that the fermentation tank (1) has a flat bottom.